Fig. 1: Structural and optical properties of NaBiS₂ NC films.

a X-ray diffraction (XRD) patterns of NaBiS₂ nanocrystals (NCs) synthesized at 80 °C and 150 °C compared with the reference pattern for the disordered rocksalt (Fm\(\bar{3}\)m) phase of NaBiS₂²⁸. b XRD patterns and photographs of NaBiS₂ NC films synthesized at 150 °C on the same day of preparation (Day 0) and after 112 days (Day 112) of storage in ambient air (60–70% relative humidity). c Absorption coefficient (α) spectrum of the NaBiS₂ NC film compared with other PV absorbers. d Spectroscopic Limited Maximum Efficiency (SLME) of various 30 nm-thick PV absorbers compared with the Shockley-Queisser limit (SQ limit, black line). The hollow and solid circles for NaBiS₂ and AgBiS₂ refer to the corresponding SLMEs without and with consideration of non-radiative losses owing to indirect bandgaps (details in Supplementary Note 1). The absorption coefficient spectrum used in (c and d) for the PV absorbers other than NaBiS₂ are extracted from refs. 14,74,75,76,77,78,79,80. Theoretical and experimental (Exp.) orbital-projected electronic density of states (DOS) for disordered Fm\(\bar{3}\)m e NaBiS₂ and f AgBiS₂. Theoretical DOS were calculated using hybrid DFT including spin-orbit interactions (HSE06+SOC) via the Special Quasirandom Structure (SQS) supercell approach. The energy of the highest occupied state is set to 0 eV. Experimental DOS were acquired from photoelectron yield spectroscopy (PYS) measurements on AgBiS₂ and NaBiS₂ NC films in the same experimental environment. The area under the partial DOS for atomic orbitals, as well as the total DOS are shaded for clarity.